Fourier Ptychographic Microscopy (FPM) is a kind of microscopy that forms an image of a specimen using Fourier Ptychographic imaging. This imaging method is based on capturing many lower resolution images under different lighting conditions, and combining them using an iterative computational process to generate a higher resolution image. Although the lower resolution images are real images, the higher resolution image is complex. FPM can achieve a high resolution and a wide field of view simultaneously without moving the specimen relative to the imaging optics.
Virtual microscopy is a technology that gives physicians the ability to navigate and observe a biological specimen at different simulated magnifications and through different two-dimensional (2D) or three-dimensional (3D) views as though they were controlling a microscope. Virtual microscopy can be achieved using a display device such as a computer monitor or tablet with access to a database of images of microscope images of the specimen. There are a number of advantages of virtual microscopy over traditional microscopy. Firstly, the specimen itself is not required at the time of viewing, thereby facilitating archiving, telemedicine and education. Virtual microscopy can also enable the processing of the specimen images to change the depth of field, and to reveal pathological features that would be otherwise difficult to observe by the eye, for example as part of a computer aided diagnosis system.
Conventional capture of images for virtual microscopy is generally performed using a high throughput slide scanner. The specimen is loaded mechanically onto a stage and moved under the microscope objective as images of different parts of the specimen are captured on a sensor. Depth and thickness information for the specimen being imaged are generally required in order to perform an efficient capture.
Any two adjacent images have a region of overlap so that the multiple images of the same specimen can be combined, or spliced, into a 2D layer or a 3D volume in a computer system attached to the microscope. Mosaicing and other software algorithms are used to register both the neighbouring images at the same depth and at different depths so that there are no defects between adjoining images to give a seamless 2D or 3D view. Virtual Microscopy is different from other image mosaicing tasks in a number of important ways. Firstly, the specimen is typically moved by the stage under the optics, rather than the optics being moved to capture different parts of the subject as would take place in a panorama. The stage movement is can be controlled very accurately and the specimen may be fixed in a substrate.
The microscope is used in a controlled environment—for example mounted on vibration isolation platform in a laboratory with a custom illumination set up so that the optical tolerances of the system (alignment and orientation of optical components and the stage) are very tight. Therefore, the coarse alignment of the captured tiles for mosaicing can be fairly accurate, the lighting even, and the transform between the tiles well represented by a rigid transform. On the other hand, the scale of certain important features of a specimen can be of the order of several pixels and the features can be densely arranged over the captured tile images. This means that the required stitching accuracy for virtual microscopy is very high. Additionally, given that the microscope can be loaded automatically and operated in batch mode, the processing throughput requirements are also high.
Fourier Ptychographic Microscopy (FPM) is an alternative to the above high throughput slide scanner. FPM can produce a 2D image of a specimen with both a high resolution and wide field of view without transverse motion of the specimen under the objective lens. This is achieved by capturing many lower resolution images of the specimen under different lighting conditions, and combining the captured images using an iterative computational process. Each iteration analyses the set of captured images sequentially to converge towards a high quality higher resolution image. The captured images are combined in the Fourier domain so that there are no image seams in real space. The ability to generate an image without discrete stitching artefacts in the spatial domain in this way is a second advantage of FPM over traditional slide scanners. A third advantage is that the generated image is complex, that is to say it includes phase information.
On the other hand, the capture of the set of images may be slow as the illumination strength may be reduced. Also, the iterative computational process can require significant processing and storage resources in order to achieve an acceptable quality. It is desirable, therefore to develop a system for FPM that is efficient and accurate.